Burner units for gas appliances

ABSTRACT

A burner unit for gas appliances, more especially gas operated refrigerators, for use in place where an explosive gas or vapour may be encountered, comprises a burner located within a casing having an air intake covered by a fine mesh, and ignition means for the burner also located within the casing but operable from outside the casing.

United States Patent [191 Sanderson et al.

1 1 BURNER UNITS FOR GAS APPLIANCES [75] Inventors: Neville EdwardSanderson; Bertram Benjamin Youngs, both of bowestoft, England [73]Assignee: British Domestic Appliances Limited, Petersborough, England[22] Filed: Dec. 4, 1973 [21] Appl. No.: 421,611

[52] US. Cl. 431/78; 62/148; 431/264; 431/346 [51] Int. Cl. F2311 5/00[58] Field of Search 431/346, 350, 264, 78, 431/80,13;62/148 [56]References Cited UNITED STATES PATENTS 2,720,851 10/1955 Strunsky .1110/119 3,006,408 10/1961 Shepherd 431/346 [45 Nov. 18, 1975 3,094,9786/1963 McCray et al. H 431/346 X 3,236,284 2/1966 Kemper 431/133,364,393 H1968 Rabe 431/264 X 3,370,436 2/1968 Romanelli 62/1483,649,156 3/1972 Conner...

3,718,423 2/1973 Matthews 431/264 Primary ExaminerEdward G. FavorsAttorney, Agent, or FirmKirschstein, Kirschstein, Ottinger & FrankABSTRACT A burner unit for gas appliances, more especially gas operatedrefrigerators, for use in place where an explosive gas or vapour may beencountered, comprises a burner located within a casing having an airintake covered by a fine mesh, and ignition means for the burner alsolocated within the casing but operable from outside the casing.

17 Claims, 8 Drawing Figures U.S. Patent Nov. 18,1975 Sheet10f43,920,375

US Patent Nov. 18, 1975 Sheet 2 of4 3,920,375

4 /4!) 42 q I t 6A 7' 8 H E-1D D1 D2 Si JQ R2 T m az i Ci Fig.4

Sheet 3 of 4 3,920,375

US. Patent Nov. 18, 1975 U.S. Patent Nov. 18, 1975 Sheet 4 of4 3,920,375

3: I Fig.6

BURNER UNITS FOR GAS APPLIANCES This invention relates to burner unitsfor gas appliances, more especially those which are required to beinstalled in places where explosive gases or vapours may be encountered,for example in motorised boats.

Thus it sometimes occurs that petrol seeps from the engine of such aboat or is accidentally spilt into the bilges. The fumes from the petrolbeing denser than air, form a layer in the bilges which may not benoticed, and if a naked flame is brought into contact with such a layerit will cause an explosion which could be very dangerous. It is nowquite common for gas-operated absorption type refrigerators to beinstalled in motorised boats, and such a refrigerator necessarilyincorporates a gas burner to provide the heat source needed to operatethe refrigerator. To reduce the risk of the burner flame coming intocontact with explosive fumes it has been proposed to have the burnerenclosed, with an air intake pipe to the enclosure extending through thehull of the boat. However such an air intake is subject to turbulence inwind which can cause the burner flame to be continually blown out. Thisproblem is only partially overcome by using a vertical pipe through thehull roof as the air inlet pipe, which in any case uses up valuablespace.

It is therefore an object of this invention to provide a safe andeffective burner unit for gas appliances, especially refrigerators,designed for use in places where explosive gas or vapour may beencountered, and which does not take up a large amount of space.

According, therefore, to the invention a burner unit for a gas appliancecomprises an enclosure, a gas burner within the enclosure, a gas inletpipe extending through a wall of the enclosure to the burner, ignitionmeans located within the enclosure and operable from outside theenclosure to ignite the burner when gas is supplied thereto, a flueoutlet for carrying away burnt gases from the enclosure, an air intakefor admitting air into the enclosure, and a fine mesh extending acrossthe air intake.

If any explosive gas or vapour should build up about and within such aburner unit, this will cause only a small explosion within the enclosurewhich, although it may extinguish the burner flame, will be preventedfrom igniting the surrounding gas or vapour by the fine mesh across theair intake, it being understood that the term fine mesh means a mesh ofsufficiently fine aperture size that the passage of a flame therethroughis effectively prevented.

Similarly the initial or re-ignition of the burner within the enclosurecan be effected without the risk of an external explosion.

Where the flue outlet is in the form of, or is connected to, a pipecarrying the gases well away from the burner unit, as will usually bethe case, we have found that there is little risk of explosive gasesentering the burner unit via the flue, but a further fine mesh can befitted across the flue if desired.

The ignition means may take the form of a spark gap located adjacent theburner, a voltage generating means such as an ignition coil, orpiezo-electric unit being provided for producing a spark between thespark gap electrodes, one of which may be provided by part of the burneritself. Alternatively the ignition means may take the form of a wirelocated adjacent the burner and heatable by the passage of an electriccur- 2 rent to a temperature sufficient to ignite gas from the burner.

The enclosure may be provided with a removable panel attachable in a gastight manner to the rest of the enclosure to provide access to theinterior thereof for servicing, the burner conveniently being carried bythe panel, which may provide substantially the whole of one wall of theenclosure. An electrode which co-operates with the burner to form aspark gap for igniting gas from the burner is then conveniently carriedby the main part of the enclosure, so that the ignition means cannot beoperated to produce a spark when the enclosure is opened by the removalof the panel carrying the burner.

Advantageously the burner unit incorporates a gas cut-off device for theburner, arranged to operate to cut off the supply of gas thereto whenthe burner flame is extinguished.

The burner may, for example, be controllable by an electrical circuithaving a starting switch which, upon closure, results in the opening ofa gas valve to supply gas to the burner and also results in operation ofthe ignition means, the circuit incorporating a temperaturecontrolledswitch operable in response to heat from the burner flame to maintainthe gas valve in the open condition despite subsequent opening of thestarting switch, and to effect the closure of the gas valve in the eventof the burner flame being extinguished.

Preferably, however, there are provided a thermally responsive actuatingdevice operable on a gas valve supplying gas to the burner to maintainit in the open condition when the burner is ignited, but which causes orpermits the valve to close when the burner flame is extinguished, andmeans for permitting gas to be supplied to the burner during operationof the ignition means and for a period sufficient to enable thethermally responsive means to open the gas valve.

Preferably the burner unit has associated with it a warning device toshow when the burner is extinguished and possibly also an indicatingdevice to indicate when the burner is functioning.

The invention is particularly applicable to burner units forgas-operated absorption type refrigerators designed for use on boats,although the invention may also be employed to advantage in other formsof gas appliances which may be required to be operated in places whereexplosive gases or vapours might be encountered.

In order that the invention may be more easily understood, two forms ofburner unit in accordance with the invention suitable for use in a gasoperated absorption type refrigerator will now be described, by way ofillustration only, with reference to FIGS. 1 to 8 of the accompanyingschematic drawings, in which:

FIG. 1 is a perspective view, in diagrammatic form, of part of thecasing of one burner unit in accordance with the invention with adetachable panel removed,

FIG. 2 is a diagrammatic view of the exterior of the detachable panel,

FIG. 3 is a sectional view of the assembled burner unit,

FIG. 4 shows electrical circuitry associated with the burner unit,

FIG. 5 is a perspective view, from below, of the second burner unit,

FIG. 6 is a part sectional plan view of this second burner unit,

FIG. 7 illustrates a modification, and

FIG. 8 is a rear view of one form of absorption type refrigeratorincorporating a burner unit in accordance with the invention.

The first burner unit illustrated in FIGS. 1 to 3 has a casing in theform of a rectangular section box comprising a main part 1A, as shown inFIG. 1, which provides the top, bottom and three sides of the casing,and a removable panel 1B, shown in FIG. 2, which provides the remainingside of the casing and carries the burner 8, the panel being securableto the main part of the casing in a gas-tight manner.

For this purpose tapped holes 1C are provided within fillets in thecorners of the main part 1A of the casing, to permit the panel 1B, whichis provided with similarly positioned holes 1E adjacent its corners, tobe clamped in position over the open side of the main part LA by screws4 (FIG. 3).

An opening 2 is provided centrally in the top of the main part of thecasing, above the burner jet, to permit burnt gases to be carried awayfrom the interior of the casing through a flue pipe 2A secured in agas-tight manner to the wall of the casing around the opening andprojecting upwards therefrom.

The base of the main part 1A of the casing has an opening 3 whichprovides the air-inlet, and this has extending across it a fine wiremesh 3A held in place in any suitable manner, for example by means of anapertured plate 313 screwed to the base of the casing as shown in FIG.3, the mesh conveniently being formed of 37 gauge 60 mesh stainlesssteel gauze.

A gas inlet pipe 6 leads, via a gas-tight screw connection 7, throughthe panel to the burner 8. A sparking plug 9 is also carried by thepanel adjacent the burner jet 8A, and voltage generating means, such asan ignition coil (not shown), is connected to the sparking plugelectrodes for generating between them a spark which is capable ofigniting gas issuing from the burner. Close to the burner jet 8A is abimetallic strip 10 carrying a contact 11 which coacts with a fixedcontact 12, and whose function will be subsequently described.

The electrical connections associated with the burner unit are arrangedas shown in FIG. 4, and are such that when a starting switch in the formof an ignition button B is depressed, thereby electrically connectingpairs of contacts B1 and B2 and B3 and B4, a bias voltage is applied tothe base of a transistor T, which bias voltage rises as the charge on acapacitor C1 increases until the capacitor becomes fully charged, and acurrent flows through a relay F; the latter then operates switches S1,S2, S3 and S4 such that $1 and S4, which are normally closed, areopened, and S2 and S3 which are normally open, are closed. Thetransistor remains biased (and hence the relay is kept operating), eventhough switch S1 is now open, by virtue of the discharge of thecapacitor CI which was charged while S1 and B were both closed. Diode D2minimises the effect of the back EMF generated by the relay F. By theoperation of button B, switch S2 is by-passed and a gas valve GVI isopened, thereby allowing gas to flow to the burner 8. The closure ofswitch S3 causes a capacitor C2 (which prevents sparking at the switchS3) to discharge, and the current increases in the primary winding of anignition coil A causing a spark at the sparking plug attached to thesecondary winding. The opening of switch S4 causes a red lamp L2 to beextinguished. When the capacitor Cl becomes discharged sufficiently sothat there is no operation bias to the transistor T, the relay ceases tofunction and the switches S1, S2, S3 and S4 return to their previousstates, and the capacitor C2- again becomes charged.

If the gas from the burner has been ignited by the spark, the heat ofthe flame results in the closure of a bimetallic switch E provided bythe contacts 11 and 12 of FIG. 1 to complete the circuit through a diodeD1 and, hence, the transistor T is biased on so that it continues toconduct. Therefore, whilst the transistor T is biased on, the relay isoperative, switches S1 and 84 are opened, and switches S2 and S3 areclosed and hence, gas flows through the gas valve GVl to burner 8, thecapacitor C2 is discharged, the red lamp L2 is extinguished and, inaddition, a green lamp L1 is lit.

If, however, the gas has not been ignited by the spark then, when theswitches S1, S2, S3 and S4 have returned to their previous states, thewhole sequence is repeated until such time as the gas is ignited oruntil the button B is returned to the off position.

Thus, the red light indicates when the gas has not been ignited, thegreen light indicates when it has been ignited, and no light at allindicates a circuitry breakdown such as battery failure.

This arrangement as described above is ideally suited to refrigerators,for use on motorised boats, where petrol fumes can build up,particularly in the bilges, and make a naked flame dangerous. Howeverwith the burner contained in the box of the burner unit, and the airinlet covered by a fine mesh as described above, there can be noflash-back from any explosion of petrol fumes within the box, and thusall that may occur is the extinguishing of the burner. The operator ofthe boat can see that lamp L1 is extinguished and that red lamp L2 islit to indicate that the burner has gone out because of an explosion (orfailure of the gas supply). In this state, of course, the relay willhave ceased to function and thus, in particular, switch S2 will be openand the gas supply to the burner will be cut off. The burner can bere-ignited as above described, again without any risk of causing a majorexplosion.

The switch E may, if desired, be replaced by a thermistor as this ismore sensitive to temperature change or by any other suitable form oftemperature sensitive device. Moreover alternative forms of startingcircuits can clearly be employed if desired.

The second burner unit illustrated in FIGS. 5 and 6 similarly comprisesa casing in the form of a rectangular section box. In this case,however, the main part 1A of the box forms the four sides and top of thebox, and a removable panel 1B carrying the burner 8 provides the bottomwall of the box, being securable to the main part 1A in a gas-tightmanner by means of four screws 4 in a similar manner to that of the unitpreviously described.

A gas inlet pipe 6 leads, via a gas-tight screw-connection 7, throughthe centre of the panel to the burner 8, an opening 2 being providedcentrally in the top wall of the casing above the burner jet, to permitburnt gases to be carried away from the interior of the casing through aflue pipe (not shown), arranged to be secured in a gas-tight manner tothe casing around the opening.

The removable panel 18, forming the base of the casing, has tworectangular openings 3 lying one on each side of the connection 7, eachof these openings having a fine wire mesh 3A extending across it asshown.

An electrode 13 is insulatingly supported by a side wall of the casingso as to terminate adjacent the burner jet 8A, and is connected via alead 14 to a piezo-electric igniter, shown schematically at 15, theburner 8 itself providing a co-operating electrode such that when thepiezoelectric igniter is operated a spark is generated betwee n theelectrode 13 and the burner sufficient to ignite gasiis ing from theburner.

A flame sen r is also supported by the side wall of the casing so as t=1 'acent the burner jet 8A, and operates directly on a ga \alvecontrolling the supply of gas to the burner and shown schematically at17.

The valve 17 is also openable by a push-button 17A to enable gas to besupplied to the burner in order that it may be ignited before the flamesensor 16 has been heated sufficiently to hold the valve open.

Thus with the button 17A of the valve 17 depressed the piezo-electricigniter is operated to generate a spark to ignite the burner, the buttonof the valve being held depressed for a short period sufficient toenable the flame sensor 16 to maintain the valve 17 in the opencondition when sufficiently heated by the burner flame.

If the burner flame should be extinguished for any reason the flamesensor 16 will cause the valve to close, the valve conveniently beingassociated with some form of a visual indicator for indicating when thevalve has moved to the closed position and thereby giving a warning thatthe burner flame has been extinguished. Such an indicator might, forexample, be provided by the push button itself, which may, in such acase, be held in when the valve is open but arranged to return to theout position when the valve is closed by the flame sensor due to theburner flame bieng extinguished, the button conveniently being brightlycoloured so that the condition of the valve, and hence of the burner, isreadily recognisable.

It will be seen that as in the previous example the presence of the finemesh 3A across the air inlet 3 ensures that should petrol fumes build uparound the burner unit any explosion will be confined to the burnercasing, and ignition of the burner, should the burner flame beextinguished, can also be effected with complete safety.

In either of the two embodiments described the fine wire mesh 3A acrossthe air inlet 3 may be augmented by a further fine wire mesh alsosupported across the inlet and spaced slightly form the first, forexample a distance of the order of one-eighth to one-fourth inch to giveadditional protection if desired, for example as shown at 3C in FIG. 7.

FIG. 8 represents diagrammatically a rear view of a gas operatedabsorption-type refrigerator fitted with a burner unit 20 in accordancewith the invention and which may be of either of the forms describedabove by way of example; the refrigerator comprises, in known manner, acondenser and evaporator shown diagrammatically at 21 and 22, and aheatable boiler 23 for supplying refrigerant in gaseous form to thecondenser. The burner unit 20 is shown clamped to the bottom of theboiler 23 with the flue opening 2 in register with the bottom of a fluepipe 2A, an asbestos gasket 24 forming a gas-tight seal between the fluepipe 2A and the burner casing. The flue pipe extends upwards through theboiler to heat it and terminates near the top of the refrigerator. Theflue outlet 25 which is located at the top and to one side of the pipemay also be fitted with a fine mesh, although since this is disposedwell above the burner unit it has been found that there is very littlerisk of explosive fumes penetrating the burner unit via the flue pipe,so that this is not usually necessary.

We claim:

1. A burner unit for a gas appliance comprising:

A. an enclosure,

B. a gas burner within the enclosure,

C. a gas inlet pipe extending through a wall of the enclosure to theburner,

D. ignition means located within the enclosure and i. operable fromoutside the enclosure to ignite the burner when gas is supplied thereto,

E. an air intake for admitting air into the enclosure F. fine mesh meansextending across the air intake and serving as a barrier which preventsthe passage of a flame therethrough.

G. a flue extending upwardly in a gas tight manner from an opening inthe top of the enclosure for carrying away burnt gases from theenclosure and H. fine mesh means extending across the flue and servingas a barrier which prevents the passage of a flame therethrough.

2. A burner unit according to claim 1 incorporating a gas cut-off devicefor the burner and operative to cut off the supply of gas thereto whenthe burner flame is extinguished.

3. A burner unit accotrding to claim 2 including an electrical controlcircuit having a starting switch which, upon closure, causes opening ofa gas valve to supply gas to the burner, and also operation of theignition means, the circuit incorporating also a temperature controlledswitch which is operable in response to heat from the burner flame tomaintain the gas valve in the open condition despite subsequent openingof the starting switch, but which allows the gas valve to close when theburner flame is extinguished.

4. A burner unit according to claim 3 wherein the temperature-controlledswitch is a bimetallic switch or a thermistor.

5. A burner unit according to claim 3 incorporating a relay energisableon closure of said starting switch, and in which closure of thetemperature-controlled switch is effective to maintain the relay in theenergised condition, the gas valve being openable on energisation of therelay.

6. A burner unit as claimed in claim 5 incorporating a transistor inseries with said relay, and in which closure of thetemperature-controlled switch is effective to maintain the transistorconductive and thereby to maintain the relay in the energised condition.

7. A burner unit as claimed in claim 5 including a relay switch in thebase line of the transistor which is openable to render the transistornon-conductive when the relay is energised unless thetemperature-controlled switch is closed, and a capacitor in the basecollector circuit of the transistor for maintaining the base bias of thetransistor for a predetermined length of time following the opening ofthe relay switch.

8. A burner unit according to claim 2 including a thermally responsiveactuating device operable on a gas valve supplying gas to the burner tomaintain it in the open condition when the burner is alight, but whichcauses or permits the valve to close when the burner flame isextinguished, and means for permitting gas to be supplied to the burnerduring operation of the ignition means to ignite the burner, and for aperiod sufficient to enable the thermally responsive actuating device toopen the gas valve.

9. A burner unit according to claim 1 in which there is an additionalfine wire mesh extending across the air intake and spaced from the firstmesh.

10. A burner unit according to claim 9 wherein the additional mesh isspaced from the first mesh a distance of the order of one-eighth toone-fourth inch.

11. A burner unit for a gas appliance comprising:

A. an enclosure,

B. a gas burner within the enclosure,

C. a gas inlet pipe extending through a wall of the enclosure to theburner,

D. ignition means located within the enclosure and i. operable fromoutside the enclosure to ignite the burner when gas is supplied thereto,

E. a flue pipe for carrying away burnt gases from the enclosure,

F. an air intake for admitting air into the enclosure,

and

G. a fine mesh of sufficiently small aperture size to prevent thepassage of a flame therethrough extending across the air intake,

H. the enclosure comprising a main part and a removable panel attachablein a gas-tight manner to the main part,

I. the ignition means being in the form of a spark gap adjacent theburner and a voltage generator operable to generate a voltage sufficientto produce a spark across the spark gap, and the spark gap comprising apair of electrodes, one of which is carried by the main part of theenclosure and the other of which is carried by the removable panel.

12. A burner unit according to claim 11 incorporating or associated witha warning or indicating device operative to show when the burner isextinguished.

13. A burner unit according to claim 11 wherein theburner itselfprovides one of the spark gap electrodes.

14. A burner unit according to claim 11 having an additional fine meshextending across the air intake, and spaced from the first mesh adistance of the order of one-eighth to one-fourth inch.

15. A burner unit according to claim 11 including a pair of spaced wiremeshes of an aperture size sufficiently small to prevent the passage ofa flame therethrough extending across the flue.

16. A burner unit according to claim 15 wherein the meshes which extendacross the flue are spaced apart a distance of the order of one-eighthto one-fourth inch.

17. A burner unit for a gas appliance comprising:

A. an enclosure,

B. a gas burner within the enclosure.

C. a gas inlet pipe extending through a wall of the enclosure to theburner,

D. ignition means located within the enclosure and i. operable fromoutside the enclosure to ignite the burner when gas is supplied thereto,

E. an air intake for admitting air into the enclosure,

F. a pair of fine mesh means each extending across the air intake andspaced from one another to serve as barriers to the passage of a flametherethrough G. a flue extending upwards in a gas tight manner from anopening in the top of the enclosure for carrying away burnt gases fromthe enclosure and H. a further fine mesh means extending across the flueto serve as a barrier to the passage of a flame therethrough.

1. A burner unit for a gas appliance comprising: A. an enclosure, B. agas burner within the enclosure, C. a gas inlet pipe extending through awall of the enclosure to the burner, D. ignition means located withinthe enclosure and i. operable from outside the enclosure to ignite theburner when gas is supplied thereto, E. an air intake for admitting airinto the enclosure F. fine mesh means extending across the air intakeand serving as a barrier which prevents the passage of a flametherethrough, G. a flue extending upwardly in a gas tight manner from anopening in the top of the enclosure for carrying away burnt gases fromthe enclosure and H. fine mesh means extending across the flue andserving as a barrier which prevents the passage of a flame therethrough.2. A burner unit according to claim 1 incorporating a gas cut-off devicefor the burner and operative to cut off the supply of gas thereto whenthe burner flame is extinguished.
 3. A burner unit according to claim 2including an electrical control circuit having a starting switch which,upon closure, causes opening of a gas valve to supply gas to the burner,and also operation of the ignition means, the circuit incorporating alsoa temperature-controlled switch which is operable in response to heatfrom the burner flame to maintain the gas valve in the open conditiondespIte subsequent opening of the starting switch, but which allows thegas valve to close when the burner flame is extinguished.
 4. A burnerunit according to claim 3 wherein the temperature-controlled switch is abimetallic switch or a thermistor.
 5. A burner unit according to claim 3incorporating a relay energisable on closure of said starting switch,and in which closure of the temperature-controlled switch is effectiveto maintain the relay in the energised condition, the gas valve beingopenable on energisation of the relay.
 6. A burner unit as claimed inclaim 5 incorporating a transistor in series with said relay, and inwhich closure of the temperature-controlled switch is effective tomaintain the transistor conductive and thereby to maintain the relay inthe energised condition.
 7. A burner unit as claimed in claim 5including a relay switch in the base line of the transistor which isopenable to render the transistor non-conductive when the relay isenergised unless the temperature-controlled switch is closed, and acapacitor in the base collector circuit of the transistor formaintaining the base bias of the transistor for a predetermined lengthof time following the opening of the relay switch.
 8. A burner unitaccording to claim 2 including a thermally responsive actuating deviceoperable on a gas valve supplying gas to the burner to maintain it inthe open condition when the burner is alight, but which causes orpermits the valve to close when the burner flame is extinguished, andmeans for permitting gas to be supplied to the burner during operationof the ignition means to ignite the burner, and for a period sufficientto enable the thermally responsive actuating device to open the gasvalve.
 9. A burner unit according to claim 1 in which there is anadditional fine wire mesh extending across the air intake and spacedfrom the first mesh.
 10. A burner unit according to claim 9 wherein theadditional mesh is spaced from the first mesh a distance of the order ofone-eighth to one-fourth inch.
 11. A burner unit for a gas appliancecomprising: A. an enclosure, B. a gas burner within the enclosure, C. agas inlet pipe extending through a wall of the enclosure to the burner,D. ignition means located within the enclosure and i. operable fromoutside the enclosure to ignite the burner when gas is supplied thereto,E. a flue pipe for carrying away burnt gases from the enclosure, F. anair intake for admitting air into the enclosure, and G. a fine mesh ofsufficiently small aperture size to prevent the passage of a flametherethrough extending across the air intake, H. the enclosurecomprising a main part and a removable panel attachable in a gas-tightmanner to the main part, I. the ignition means being in the form of aspark gap adjacent the burner and a voltage generator operable togenerate a voltage sufficient to produce a spark across the spark gap,and the spark gap comprising a pair of electrodes, one of which iscarried by the main part of the enclosure and the other of which iscarried by the removable panel.
 12. A burner unit according to claim 11incorporating or associated with a warning or indicating deviceoperative to show when the burner is extinguished.
 13. A burner unitaccording to claim 11 wherein the burner itself provides one of thespark gap electrodes.
 14. A burner unit according to claim 11 having anadditional fine mesh extending across the air intake, and spaced fromthe first mesh a distance of the order of one-eighth to one-fourth inch.15. A burner unit according to claim 11 including a pair of spaced wiremeshes of an aperture size sufficiently small to prevent the passage ofa flame therethrough extending across the flue.
 16. A burner unitaccording to claim 15 wherein the meshes which extend across the flueare spaced apart a distance of the order of one-eighth to one-fourthinch.
 17. A burner unit for a gas appliance coMprising: A. an enclosure,B. a gas burner within the enclosure. C. a gas inlet pipe extendingthrough a wall of the enclosure to the burner, D. ignition means locatedwithin the enclosure and i. operable from outside the enclosure toignite the burner when gas is supplied thereto, E. an air intake foradmitting air into the enclosure, F. a pair of fine mesh means eachextending across the air intake and spaced from one another to serve asbarriers to the passage of a flame therethrough G. a flue extendingupwards in a gas tight manner from an opening in the top of theenclosure for carrying away burnt gases from the enclosure and H. afurther fine mesh means extending across the flue to serve as a barrierto the passage of a flame therethrough.